Demand for efficient and reliable electrical power is escalating, outpacing the improvements in conventional power sources. Applications in which compact, lightweight, energy-dense power supplies would find immediate application include portable power generators, combined heat and power systems, and auxiliary power units for vehicles. Concerns about global climate change are increasing with the level of CO2 in our atmosphere caused by the use of combustion based methods to generate power from fossil fuels. Fuel cells offer a viable approach to increase efficiency of power generation from fossil fuels while greatly reducing emissions of pollutants and greenhouse gases. Of the various types of fuel cells, the proton exchange membrane (PEM) fuel cell, which operates with hydrogen as a fuel, has received considerable attention due to its low weight, low-temperature operation, and ease of manufacture. However, the operation of PEM fuel cells with fossil-based hydrocarbon fuels requires extensive pre-processing (reforming) to convert the hydrocarbons into a hydrogen rich gas and subsequent gas purification steps to reduce carbon monoxide and sulfur to very low levels (CO<10 ppm and H2S<10 ppb). Solid oxide fuel cells (SOFCs), which operate at high temperature (typically, 600 to 1000° C.), are much less sensitive to impurities in hydrocarbon fuels, which minimizes the amount of gas purification steps required. This greatly increases power generation efficiency and reduces system complexity. It also is theoretically possible to operate solid oxide fuel cells directly on certain hydrocarbon fuels (e.g., methane, methanol and ethanol) via internal reforming, i.e., without an initial reforming step.
A solid oxide fuel cell is comprised of an oxygen ion conducting ceramic electrolyte membrane that is sandwiched by a fuel electrode (anode) and an air electrode (cathode). Power is generated by passing air (or oxygen) over the cathode and fuel (e.g., hydrogen plus carbon monoxide) over the anode and collecting the electrical current that is created by the electrochemical reaction of oxygen with fuel to form steam and carbon dioxide. The key to successful development of SOFC systems is the electrochemical cell design and “stacking” configuration (i.e., the manner in which SOFC elements are configured within a power producing SOFC module). In this regard, it is important to pack a large amount of active area for electrochemical reactions within the smallest volume possible. A typical solid oxide fuel cell will generate about 30 to 40 watts of power for every 100 cm2 of active area—this translates to more than about 3000 cm2 of active area for a kilowatt of power. Another key is maximizing the electrical efficiency of power generation (defined as the output power divided by the lower heating value of the input fuel). A reasonable target for commercially viable systems is electrical efficiency of greater than approximately 50 percent. This requires that most of the fuel fed to the SOFC be used to generate power. The various cell and stack design alternatives are discussed in the following paragraphs.
Tubular solid oxide fuel cells include a multi-layer tube fabricated with cathode, electrolyte, anode layers, and in some cases interconnect layers. Tubular SOFCs may be supported by anode, cathode, or electrolyte materials or a porous inert and electrically insulating material with subsequently deposited thin-film anode, electrolyte, cathode and interconnect layers. Conventional tubular cells typically suffer from low volumetric or gravimetric power density because large tubes do not pack well and have a low surface area to volume ratio. Power densities achievable with conventional tubular cells also are limited by the long current collection paths intrinsic to long-length tubular cells. Microtubular SOFCs, typically with diameters of less than about 5 mm, overcome some of the disadvantages of conventional tubular fuel cells. Sealing of small diameter microtubes is simpler than sealing of conventional tubes. Microtubular cells also overcome the low surface area to volume ratio associated with conventional tubular cells. However, microtubular cells require complex manifolding and electrical interconnection schemes so that scaling to higher powers (more than about 100 watts) is difficult.
Planar SOFCs may be supported by either the anode material or the electrolyte material, also have been demonstrated extensively. Anode-supported cells often are preferred because these cells can accommodate a thin electrolyte layer (less than about 20 microns). This reduces electrolyte ohmic resistance in the cell and allows operation at relatively low temperatures (e.g., T<800° C.). Planar anode-supported cells are particularly attractive for mass market, cost-driven applications because of their high areal power density and their advantageous packing efficiency. Performance of anode-supported cells at 700° C. has been demonstrated to be over 1 W/cm2 in small cells at low fuel use. With appropriate seal and interconnect technology, power densities greater than 0.4 W/cm2 have been reported for anode-supported cell stacks. However, anode-supported cells are not without drawbacks. When conventional nickel oxide/yttrium-stabilized zirconia (NiO/YSZ) composites are used as support materials, the reduction of NiO to nickel metal creates stress in the electrolyte layer, which may result in considerable deformation of the support. Operating planar anode-supported cells at high power density and high fuel use also is difficult; the thick porous layer prevents rapid diffusion of steam away from the electrolyte and results in increased cell area-specific resistance (ASR) at high current density.
Electrolyte-supported planar cells have an electrolyte layer that provides the mechanical strength of the cell. Electrolyte-supported cells offer numerous advantages in the production of SOFCs. The sealing of electrolyte-supported cells is simpler than electrode-supported planar cells because a dense electrolyte perimeter can be preserved during cell processing, which provides a dense, smooth surface for sealing operations. Electrolyte-supported cells also have good stability during reduction. Because only a thin layer of anode is affected by the reduction process, this process generally has little impact on cell mechanical stability. The gas diffusion path in and out of the thinner anode layer is short, making fuel and steam diffusion limitations less of a concern. However, under identical operating conditions, conventional electrolyte-supported cells often exhibit much higher area-specific resistance values than anode-supported cells because the electrolyte is more resistive than the anode or cathode materials. To compensate for this higher area-specific resistance, the operating temperature for electrolyte-supported cells generally is higher than anode-supported cells using the same materials set. The higher operating temperature of the electrolyte-supported cells can be a drawback, particularly for developers wishing to use metallic interconnect materials.
Two recent U.S. patents, U.S. Pat. No. 8,192,888 (issued Jun. 5, 2012) and U.S. Pat. No. 7,736,787 (issued Ser. No. 11/220,361 (issued Jun. 15, 2010), owned by NexTech Materials, Ltd., describe novel planar cell structures that overcome technical barriers associated with building SOFC stacks with conventional planar cells. The first of these, referred to as the FlexCell, comprises a thin electrolyte membrane layer that is mechanically supported by a “honeycomb” mesh layer of electrolyte material (see FIGS. 1 and 2). More than 75 percent of the electrolyte membrane within the active area of the FlexCell is thin (20-40 microns) and the periphery of the cell is dense. Electrode (anode and cathode) layers are subsequently deposited onto the major faces within the active cell regions to complete the fabrication of an SOFC based on the FlexCell structure. The second structure, referred to as the HybridCell, comprises an anode layer (30-40 microns) co-sintered between the mesh support layer and the electrolyte membrane layer (see FIGS. 3 and 4). The entire active cell area of the HybridCell has a thin electrolyte membrane (10-20 microns) and the periphery of the cell is dense. The cathode layers are subsequently deposited onto the major faces within the active cell region to complete the fabrication of an SOFC based on the HybridCell structure. Specific advantages of these two types of cells are summarized below:
Thin Electrolyte Membrane Layers for High Performance. Both the FlexCell and HybridCell feature a thin electrolyte membrane layer (10-40 microns), which minimizes electrolyte ohmic losses at lower operating temperatures. Thus, SOFC performance levels achieved with these cells can be made equivalent to those of anode supported cells.
Small Repeat Units for High Power Density. The total thickness of the FlexCell and HybridCell are less than 200 microns after deposition of electrodes, which compares to more than 600-1000 microns thickness of conventional anode supported cells. This greatly reduces size and weight contributions of the cells to the total stack weight and volume. Thus, high power density SOFC stacks can be constructed.
Mechanical Strength and Flexibility. The use of partially stabilized zirconia (yttria or scandia doped) as the mesh support component of the cell results in high mechanical strength, which makes the cell easier to handle during stack fabrication operations. This also reduces the amount of pressurization required during stack operation because pressure is required only to maintain gas-tight seals and not to keep the cells flat (as is the case with anode supported cells).
Dense Perimeter for Ease of Sealing. The dense electrolyte perimeter of the FlexCell and HybridCell structures aids sealing. The sealing surfaces are dense and made of a relatively inert electrolyte material rather than porous and made of a relatively reactive electrode material.
Thin Anode for Redox and Thermal Cycling Tolerance. The thin anode layer intrinsic to the FlexCell and HybridCell structures makes it much easier to produce cells that are tolerant to both redox and thermal cycling. Thus, excessive measures are not required to maintain the anode in its fully reduced state during transient operation of SOFC stacks.
Anode Material Flexibility. A specific advantage of the FlexCell design is that the anodes are deposited in a separate manufacturing operation. This makes it easy to incorporate new anode materials that provide greater functionality (e.g., anodes that are tolerant to sulfur impurities).
For SOFCs to be of practical application, they must operate using fuels that are easily available. This requires that power supplies operate on conventional fuels, such as gasoline, natural gas, and diesel. The hydrocarbon fuel is pre-reacted (reformed) over a catalyst with air and/or steam to produce a mixture of H2 and CO (and in some cases CH4) gas before delivery to the fuel cell. Promising development is underway to provide compact and lightweight reformers for conventional fuels. However, traditional fuels contain some level of sulfur. Sulfur can have devastating effects on conventional SOFC performance. Cermet mixtures of nickel metal with electrolyte materials (YSZ or GDC) are the most common SOFC anodes, but are susceptible to sulfur poisoning in concentrations as low as a few ppm. This leads to significant performance degradation, especially at lower operating temperatures (700-800° C.) which are desired for SOFC stacks that use inexpensive metallic interconnect components. Nickel-based cermet anodes experience a two-stage deactivation when exposed to sulfur (see FIG. 5). The following mechanisms have been proposed:
Stage I Degradation: The first stage of anode degradation is characterized by a rapid drop in cell performance upon introduction of sulfur to the fuel and is nearly instantaneous. Stage I degradation is largely recoverable upon removal of the sulfur contaminant. The Stage I degradation process is theorized to be related to the coverage of active nickel sites (at triple-phase boundary regions) with sulfide. The reactions that occur in Ni/YSZ anodes are shown below:NiO+H2Ni+H2O ΔG at 750° C.=−44 kJ/mol  (1)3Ni+2H2SNi3S2+2H2 ΔG at 750° C.=−68 kJ/mol  (2)While bulk nickel sulfide formation is not favored in low concentrations of H2S (<2000 ppm), sulfidation of small nickel particles and rough surfaces does proceed readily even in very low H2S concentrations. Surface sulfidation (but not in the bulk) of nickel to Ni3S2 has been observed experimentally with Raman spectroscopy by analyzing a Ni/YSZ cermet exposed to 100 ppm H2S.
Stage II Degradation: The second stage is characterized by a slower degradation of cell performance, which is not recoverable. Some researchers have reported a cascading effect at longer times. The mechanism of this degradation is theorized to be due to a loss of nickel surface area through surface rearrangement and sintering of the nickel particles. Nickel sulfide (Ni3S2) has a melting point of 787° C.; thus, sulfide formation could contribute extensively to nickel sintering.
Desulfurizers are being developed to protect fuel cell anodes from sulfur but they are too large, heavy and complex for many applications, accounting for 10-50% of total system weight, depending on the targeted fuel and useful desulfurizer life. Additionally, desulfurizers add cost and maintenance requirements to SOFC systems. An ideal SOFC system would tolerate sulfur without the cost, weight, and volume of the desulfurizer although for certain applications inclusion of a desulfurizer still may be preferred. In such systems, the capability of SOFC anodes to resist degradation by sulfur will offer an opportunity to minimize the durations between replacement of sulfur adsorbent beds, so that all of the desulfurizer capacity can be used and service costs reduced. Sulfur tolerant SOFC anodes therefore are a critical and enabling technology need. One embodiment of the present invention is an entirely new approach to achieving sulfur tolerance in solid oxide fuel cells. A novel anode materials system, based on commonly used SOFC materials and methods, provides the unique capability to achieve sulfur tolerance in SOFCs without sacrificing power density, resorting to excessively high SOFC operating temperatures, or adding significant cost.
A key to controlling cost in planar solid oxide fuel cell stacks is the use of low-cost metals for the interconnect components. In addition to low cost, there are a number of technical requirements of metallic interconnect materials, including but not limited to thermal expansion match with SOFC materials, high oxidation resistance at elevated temperatures in oxidizing environments, and the ability to use low cost fabrication methods (e.g., rolling and stamping) to fabricate interconnect materials of desired shapes. Many alloys have been evaluated but only a few have been shown to possess the desired properties. In particular, chromium-containing ferritic alloys (e.g., Crofer 22-APU, E-Brite, SS-441, and others known in the art) have shown promise for SOFC applications. Although these alloys are not completely immune to oxidation at SOFC operating temperatures (i.e., on the cathode faces), the scale that forms during oxidation is itself electrically conducting. However, these alloys do show a tendency for chrome evaporation during SOFC operation, with chrome migrating to the cathode material and causing deterioration of cathode performance. For this reason, considerable effort has been focused on the development of coatings that could be applied to the cathode faces of the metallic interconnects for the dual purposes of further suppressing alloy oxidation or suppressing migration of the volatilized chrome species. Although progress is being made in development of such coatings, another embodiment of this invention is a novel cathode materials system for stabilizing cathode materials against the deleterious effect of chrome poisoning.